Method for preparing flat-bottom thermoplastic sack

ABSTRACT

A method for preparing flat bottom thermoplastic sacks comprising process steps for forming a tube of thermoplastic film, collapsing said tube while forming two oppositely disposed gussets therein, forming two pairs of diagonal sealed seams in the gussets, forming two pairs of diagonal preweakened lines parallel to said diagonal seals, forming a transverse sealed seam across the tube along a line which includes the inboard ends of the diagonal seams and forming preweakened transverse lines closely adjacent and parallel to said transverse sealed seam or forming a severing line along this line, removing the four double triangular regions bounded by the diagonal preweakened lines, the transverse preweakened lines and the side edges of the tube and collecting the resulting structures either while still interconnected or stacking the severed structures and then removing the triangular regions. The final structure can have handles or it can be handleless.

The present invention relates a method for preparing a thermoplasticsack, and, more particularly, to a method for preparing such a sackhaving at least a substantially flat rectangular bottom. One type of bagcontemplated for preparation by a process of the present invention ismore particularly defined in copending U.S. patent application Ser. No.606,120, filed May 2, 1984, now U.S. Pat. No. 4,554,192, the disclosureof which is in its entirety incorporated by reference herein.

The bag structure of said patent application is of a thermoplastic filmmaterial and it comprises front and rear bag walls connected by gussetedside walls. The bag has an open mouth top portion, which ischaracterized by having handles located at opposite end regions thereof,said handles each being of two films as a result of being integralextensions of said front, rear and gusseted side walls. The bag has abottom wall planarly extensible so as to form a rectangle with at leastno substantial excess film outside of the bulk volumetric capacity ofthe bottom region of said bag. An alternative manner of describing thebottom of such a bag is that said bottom is formed from integralextensions of said front, rear and side walls and the closure thereof is4 two-film gusset-to-wall heat seals, when said bottom is a square, and4 two-film gusset-to-wall heat seals and 1 two-film front wall-to-backwall heat seals when said bottom is a rectangle. The present inventionalso contemplates forming the same type of bag without handles.

It is an object of the invention to present a process for thepreparation of the above-defined flat bottom sack.

It is yet another object of the invention to present a process forpreparing flat bottomed handled sacks in bag pack form.

SUMMARY OF THE INVENTION

The present invention is concerned with a method for continuouslypreparing a stack of at least substantially flat, rectangular-bottom,thermoplastic film sacks comprising:

(a) forming a tube of thermoplastic film;

(b) collapsing said tube while simultaneously forming therein twooppositely disposed, parallel gussets;

(c) forming two pairs of diagonal sealed seams over the portions of thetube width corresponding to the gussets along lines diagonal to thelength of the tube, the seal pairs of opposite sides being at an angleto one another and the seams of each pair being in registration with oneanother;

(d) simultaneously or spacially sequentially forming closely parallel tosaid diagonal sealed seams a diagonal preweakened line in said film;

(e) forming a transverse sealed seam across said tube along a line whichincludes the inboard ends of said pairs of diagonal seams andsimultaneously forming a preweakened transverse line closely adjacentand parallel to said transverse sealed seam;

(f) individually separating these structures along said preweakenedtransverse line and forming a stack of them in registration; and

(g) removing from each bag of the stack the four double film triangularregions bounded by said diagonal preweakened lines, said transverse seamand the side edges of said tube.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view, in schematic form illustating a system andprocess sequence of one form of the present invention.

FIG. 2 is a plan view of another form of the process of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

As indicated in FIG. 1, 10 illustrates a system and process sequence forforming a handled, gusseted, flat bottomed thermoplastic sack. A tube ofthermoplasic film 12 is in the process of being collapsed, by beingpassed through a gusset forming means 14.

The tube collapsing and gusset-forming means 14 can be any devicesuitable for accomplishing these two steps substantially simultaneously.A typical gusset forming device is a open-ended box shaped arrangementof, for example, sheet metal material, which has two inwardly extendingmembers which are brought to bear on the collapsing thermoplastic tubeso as to form two oppositely disposed inwardly extending folds orgussets. The device gradually tapers to cause the gusseted tube tocollapse completely. These gussets are illustrated in the drawing bydotted lines 16. The gusset forming means 14 can be prepared in aplurality of different size gusset forming means so as to permitflexibility in the dimension of the resulting gusset in-fold. The gussetforming means can be such as to impress a gusset in a tube of anywherefrom a fraction of an inch up to that providing gussets which extend allthe way to the midline of a fully collapsed tube. The gusseted portionof the tube therafter traverses two Teflon coated plates 18, (or someother suitable non-stick separating surface) each of which extend intothe full reach of the gusset 16. Thereafter, a sealed seam is impressedin each of the four gusset folds as shown at 20. Simultaneously a lineof perforations 21 is formed parallel to each seal seam 20. This willsubsequently permit the removal of all corners from a stack of bagswithout there being any film-loss or bag destruction due tomisregistration during stacking. The Teflon plate prevents one seam fromfusing to the other on each side of the tube. These seams are bestformed by heat sealing members which are Teflon coated resistance wiresheated to a temperature which will permit the two layers of each of thefour gusset folds to be heat seal together but not severed. Thesediagonal sealed seams extend over the portions of the tube widthcorresponding to the gussets along lines diagonal to the length of thetube and the seal pairs of opposite sides are at an angle to oneanother, preferably at an angle of at least approximately 90°. The seamsof each pair of seams on each side of the gusseted tube are inregistration with one another. In other words, beneath seam 20, and onthe other side of Teflon separator plate 18, there is an identical seal20. The diagonal line of perforations parallel to the diagonal seams mayalso be lines of reduced thickness. The perforations or reducedthickness can be formed by any piercing or punching means or by anyscoring means.

As the tube, seals 20 and perforations 21 progress through the systemthere is subsequently formed a bottom seal 22 which simultaneously sealsand severs the bottom of the gusseted structure. This is accomplished bymeans of bottom sealing and severing device 24. Seal line 22 is atransverse sealing and severing seam which extends across and throughthe collapsed tube along a line which includes the inboard ends 26 ofeach pair of diagonal seams. Transverse sealing and severing device 24can be any suitable means which will bond and sever the collapsed tubealong the defined line. This sealing and severing action will isolatesack blank 28 from the collapsed and gusseted tube. Sack blank 28 willhave a seal 30 across the top thereof and 4 triangular regions 32 willbe bounded by perforation lines 21, 26 and the side edge of the gussetedstructure. The sack blank structures 28 may be separately orcollectively converted into bag structures by removal of the triangularregions 32 and by the formation of bag mouth opening 34 whichsimulataneously forms handle loops 36, each of which are formed of twolayers of film by virtue of the gusseted structure. This will completethe formation of a handled sack, the bottom of which can be planarlyextended so as to have a flat bottom. By "planarly extended" is meantthat the bag is extended in internal volume to its utmost by expandingthe gussets fully, thereby forming a flat bottom, rectangular in nature,having a length larger than its width.

In a preferred embodiment of the process illustrated in FIG. 1 of thedrawing, a linear low density polyethylene tubular film 12, of about 1mil in thickness, is collapsed and simultaneously gusseted by passingthrough gusset forming means 14. This action infolds the collapsed tubeto yield gussets 16. As the gusseted tube progresses through the system,seals 20 and 26 and perforation lines 21 are simultaneously formed. Thisaction produces bottom seal 26 and top seal 30 at the same time. Thisalso isolates sack blank 28. Sack blank 28 can then be removed to astation which can separately remove triangular sections 32 alongperforation lines 21 and remove film to form bag mouth opening 34 andform handles 36 on individual blanks. Alternatively, the blanks 28 canbe stacked in vertical registration and, through the means of a suitableremoving mechanism, e.g., a clamp, shear, etc., the triangular regionsare removed and the handle and bag mouth opening can be formed. Atypical bag can have a bag front and rear wall width of about 12 inches,4 inch gusset folds, yielding 8 inch side walls and a bag length ofabout 20-25 inches with or without handles.

For the formation of bags which do not contain a handle, the process andthe system are as shown in FIG. 2. The process 10a is different fromthat of FIG. 1 beginning at the point where the bottom of the bag issealed. Thus, a bottom seal and perforating mechanism 38 is shownimpressing seal seam 40 in the flattened gusseted tube and, simultaneoustherewith, perforation line 42 is formed closely adjacent and parallelto seal line 40. Seal line 40 does not sever the tube, merely seals it.Seal line 40 and perforation line 42 are impressed in the gusseted tubeat the same time diagonal seals 20 and diagonal perforation line 21 areimpressed into the gussets. Diagonal perforation line 21 and transverseperforation line 42 touch or intersect for easy removal of the fourtriangular regions 44. After such removal, this will yield sack 46attached by perforation line 42 to a like structure. Thereafter, thesesacks may be rolled upon themselves to form a cylindrical package,collected in a zig-zag folded stack arrangement or individuallyseparated and folded or stacked. Either structure will permit a sack tobe torn free along the perforation lines to yield an open mouth bagwhich when extended fully to the total width of the gusset, will yield aflat bottomed sack. The sack resulting from the process of FIG. 2 willhave a rectangular bottom with a length larger than its width.

In a modification of the process shown in FIG. 2, open mouth handlelessstructures can be formed by severing each structure along transverseperforation line 42, stacking the structures with the triangular cornersstill attached and thereafter removal of all corners at once alongdiagonal perforation lines 21.

Handled bags of the structure described can be formed with a pair ofregistered tabs as integral extensions of the bag mouth as shown at 39in FIG. 1. The bags can be stacked in registration to contain 50, 100,150, etc., bags and the tabs 39 fastened together in some suitablemanner. For example, the tabs can be ultrasonically welded together. Aperforation line across the tab will form a convenient tear-off site forindividual bags to be removed from the bag pack.

Bags of the structures described can be formed by employing any suitablethermoplastic material, for example, polyolefins. In employing the term"polyolefins" its employed generically to include all forms of thepolymer species including, for example, low density polyethylene, linearlow density copolymers of ethylene and another alpha olefin, highdensity polyethylene, mixtures and blends of the same, and other monomercounterparts, etc. Thermoplastic coated paper stock is alsocontemplated. The bag film can be of any gauge, for example, from about0.3 to about 5 mils in thickness.

In forming the flat bottomed bag, it is preferred that the angle betweenthe diagonal seal line and the bottom seal be at least approximately135° when said bag is in its lay flat condition. It is also preferredthat the open mouth portion of the handled sacks of the presentinvention have stress relief curves or arcs extending from the base ofthe handle to a raised portion of the bag mouth. This will militateagainst splitting or tears occurring in the bag mouth during handleextension or when the bag is being filled with bulky material. Thehandles and bag mouth opening can have any configuration.

In forming the bag structure the pertinent angles need not be precise,e.g. exactly 90°, 45° or 135°. Reasonable processing latitude shouldpermit some deviation from these guidelines. Likewise, in forming sealseams, the ends thereof can be curved or angled in order to accommodateor insure good closure at all film interfaces and joints. Double orplural line seams and seals are also contemplated.

As disclosed, the bag structure formed by the process illustrated inFIGS. 1 and 2 will yield a sack having a rectangular bottom whenplanarly extended, which bottom will have a length longer than itswidth. In other words, the sack bottom will not be a square. In order toform a square bottom sack either with or without handles the process andmeans merely need be modified so that the gusset folds are both infoldedto the maximum. That is, the collapsed tube will be gusseted to thelongitudinal mid-line of the tube. In this case the center-fold of eachgusset will contact the other and the angle between the thereafterformed diagonal seams will be at least about 90°. The resulting sackwill then have a substantially flat square bottom when planarlyextended.

It is to be understood that the preweakened lines described herein canbe lines of perforations, lines of reduced thickness, such as scorelines, etc. It is also to be understood that the diagonal preweakenedlines can be formed prior to, simultaneously with or after the formationof the diagonal seals. This will permit use of the methods described incopending application Ser. No. 606,319, filed May 2, 1984 and Ser. No.606,320, filed May 2, 1984. The disclosures of these applications areincorporated herein in their entirety.

What is claimed is:
 1. A method of continuously preparing an at leastsubstantially flat, rectangular-bottom, thermoplastic film sackcomprising:(a) forming a tube of thermoplastic film; (b) collapsing saidtube while simultaneously forming therein two oppositely disposed,parallel gussets; (c) forming two pairs of diagonal sealed seams overthe portions of the tube width corresponding to the gussets along linesdiagonal to the length of the tube, the seal pairs of opposite sidesbeing at an angle to one another and the seams of each pair being inregistration with one another; (d) simultaneously or spaciallysequentially forming closely parallel to the line of said diagonalsealed seams a preweakened line in said film; (e) forming a transversesealed seam across said tube along a line which includes the inboardends of said pairs of diagonal seams and simultaneously forming apreweakened transverse line closely adjacent and parallel to saidtransverse sealed seam; (f) individually separating these structuresalong said preweakened transverse line and stacking them inregistration; and (g) removing from each corner of the stack all thefour double film triangular regions bounded by said preweakened linesparallel to said diagonal seam, said transverse seam and the side edgesof said tube.
 2. The method of claim 1 wherein said diagonal preweakenedlines are formed prior to said diagonal sealed seams.
 3. The method ofclaim 1 wherein said diagonal preweakened lines are formed after saiddiagonal sealed seams.
 4. The method of claim 1 wherein said preweakenedlines are lines of perforations.
 5. The method of claim 1 wherein saidpreweakened lines are lines of reduced thickness.
 6. The method of claim1 wherein said thermoplastic film is of a polyolefin.
 7. The method ofclaim 1 wherein each gusset is equal and the centerfold of each gussetis spaced one from the other and the angle between projected diagonalseam lines is at least about 90° so that the resulting sack has an atleast substanially flat rectangular bottom of unequal dimensions whenplanarly extended.
 8. The method of claim 1 wherein simultaneously withforming said preweakened transverse line, a second transverse sealedseam is formed across said tube parallel to said preweakened transverseline so that said preweakened line is between the closely spacedtransverse sealed seams.
 9. The method of claim 8 wherein simultaneouslywith or subsequent to the removal of the four folded triangles, forminga handle and sack mouth opening at the opposite end of said stackedstructure and bonding said stack together.